perf(network): retain raw block bodies in reorder backlog

[evan-forbes]

Motivation

This is the third block-sync PR in the stack. It keeps raw body bytes available in the reorder backlog while preserving the existing external apply/commit driver shape.

Solution

• Thread raw body data through the wire/peer routine/sequencer path where the existing external apply flow can still use it.
• Keep the already-decoded block on the immediate contiguous path.
• Retain raw frame payload bytes only for non-contiguous reorder backlog so delayed backlog does not hold decoded blocks.
• Add a direct sequencer test for raw-byte retention across a delayed backlog drain.

Scope boundary:

• No sequencer-owned apply refactor.
• No block_sync/apply_item.rs, zebrad committer ownership, or BlockApplyExecutor changes.
• Congestion and parameter tuning are stacked above this PR.

Tests

Passed locally on the rebuilt split stack:

• git diff --check origin/review/headersync-roots..review/blocksync-throughput-defaults
• cargo fmt --all -- --check
• cargo test -p zebra-network zakura::block_sync --lib (146 passed on the final stack tip)
• cargo test -p zebra-network sequencer_retains_raw_bytes_for_non_contiguous_backlog --lib

Attempted but blocked by the local toolchain before test execution:

• cargo test -p zebrad --test acceptance latest_config_is_stored -- --nocapture
• Blocker: bundled librocksdb-sys C++ compilation fails on RocksDB headers using uint64_t without <cstdint>.

Specifications & References

Stack context:

• Base: perf(network): reduce block sync request bookkeeping #285 / review/blocksync-request-bookkeeping

Follow-up Work

Floor-priority scheduling, congestion control, and throughput-default tuning are separate stacked PRs above this one. Sequencer-owned apply remains out of this stack slice.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: Codex was used to reconstruct the stacked branch, run sequential review agents, fix review findings, draft/update PR text, and run validation commands.

PR Checklist

• The PR title follows conventional commits format: type(scope): description
• The PR follows the contribution guidelines.
• This change was discussed in an issue or with the team beforehand.
• The solution is tested.
• The documentation and changelogs are up to date.